Why does he disappear when I go out with friends?

Like we actually talk all day every day for about 6 months now. Distance is really the only thing keeping us apart.

Whenever he goes out he usually sends me photos and tells me about it. Even messages me when he's going home almost like to let me know he'll be around.

Whenever I go out or have plans he seems odd. There's been a few times he hasn't but usually he is.

He will leave my messages unread! Either for hours or until the next day. Sometimes he seems cold.

He asked me if I was lookin forward to a Halloween party tonight. I told him a little because I'm sick. He just leaves my message unread.

I am assuming he doesn't want to see how I look tonight or anything. (And no, no "sexy" costume).

So I feel a bit bummed I can't share my night with him. Clearly he's not interested to see. Should I anyway? And why does he do this? Is it hard because he has feelings for me and wants to be there? Or thinks someone else will snag me? Honestly it's the only guesses I have.